Primary developmental language impairment (LI) affects over 3 million children in the U.S., many of whom are disadvantaged socially and academically by small and shallow vocabularies. Currently, little is known about how the word learning processes of people with LI changes over development. This critical gap in the knowledge base prevents the design of vocabulary interventions that are developmentally appropriate. The design of such interventions is the long-term goal. The current objective is to discover how the word learning of children with LI changes over developmental time. The central hypothesis is that the challenge of word learning at different ages varies with the word-learning situation, the component of the word to be learned, and the development of underlying cognitive mechanisms. The project designed to test this hypothesis compares children with LI to age-mates with normal language development as they learn and retain new words over the course of one week during each of four years beginning at age six. This approach is innovative because it departs from the status quo by examining learning longitudinally. The project involves two specific aims: 1) to establish a developmental trajectory of word learning associated with LI that determines a) how learning and development vary with the learning situation and b) how learning and development vary with the component of the word to be learned; and 2) to specify the cognitive mechanisms underlying this developmental trajectory. The expected contribution is a more nuanced and comprehensive understanding of the word learning challenges associated with LI and how these challenges change during a crucial period, the first years of formal schooling. This contribution is significant because it is expected to advance and expand understanding of the dynamic interaction between word learning and cognitive development. Vocabulary interventions are not currently tailored to focus on the most critical aspects of the problem at the given developmental level of the client with LI because of ignorance about what is critical and when. By addressing this issue, this project will offer new possibilities for tailoring preventive and therapeutic interventions in light of the child's developmental needs.